Torchwood: Sigil

Sam has been haunted for years by the Sigil. She’s drawn the pattern since she was a child, she has a tattoo of it, she sells prints of it. Then one day, she sees it on a statue in the window of Bilis Manger's shop. It turns out the Sigil is very old indeed.

Why has it been haunting her? Why is it on a Celtic statue of a bird? Where did it come from? Has a terrible entity been released from its ancient slumber?